The present invention relates to a braking pressure control device for an antilocking automotive vehicle brake system. Such systems typically include a braking pressure generator, at least one wheel brake cylinder connected via a brake line with the braking pressure generator to actuate a brake, a braking pressure control circuit which is interposed into the brake line between the braking pressure generator and the wheel brake cylinder. Such systems have a piston pump, the outlet of which is connectible to the brake line, and an inlet control valve controllable by a control device to connect the wheel brake cylinder with the brake line and an outlet valve connected in a return line leading to the inlet of the piston pump. The piston of the piston pump is axially movable by a mechanical eccentric drive and is sealed in relation to the cylinder bore in the pump housing by means of a slide seal.
Braking pressure control devices of this type automatically control the braking pressure in dependence on the rotational movement of one or more of the wheels of a vehicle, which movement is monitored by a monitoring device in order to prevent locking of the wheels during a braking operation and, in case of need, also to prevent spinning of the driven wheels on starting to drive.
A braking pressure control device of the type referred to hereinabove is taught by U.S. Pat. No. 4,715,666. To reduce the braking pressure in the wheel brake cylinder with a braking pressure control device of this type, the brake line leading to the wheel brake cylinder is connected to the return line by opening of an electromagnetic control outlet valve, the return line being in direct communication with the inlet of the piston pump without the intermediary of a reservoir open towards the atmosphere. The piston pump supplies the pressure fluid out of the return line back into the brake line in order to thereby re-increase the braking pressure on the wheel brake cylinder after a renewed closing of the control outlet valve. The connection between the pump outlet and the wheel brake cylinder is governed by a pressure-controlled flow control valve which interrupts or throttles the pressure fluid supply from the piston pump to the wheel brake cylinder when the control valve is open. A low-pressure accumulator connected to the return line has a spring-loaded piston which ensures that the speed of pressure decrease does not depend on the delivery rate of the piston pump.
The piston pump of the known braking pressure control device described in U.S. Pat. No. 4,715,666 comprises a stepped piston which isolates, with a large-diameter piston step, the working chamber that is adjacent to the piston end from a suction chamber which surrounds a smaller-diameter piston step, and to which the return line is connected. The working chamber and the suction chamber are interconnectible by an inlet valve arranged in the stepped piston. Towards the drive side of the stepped piston, the inlet chamber is bounded by an annular piston received over the stepped piston and engaged with a spring on a spring retainer formed on the stepped piston. Another spring is arranged between the spring retainer and the housing. The resiliently supported annular piston causes the suction chamber to function as a low-pressure accumulator which receives pressure fluid in excess when the inlet valve opens. The springs by which the annular piston and the stepped piston are supported on one another and on the pump housing are dimensioned such that the stepped piston moves into operative engagement with the drive eccentric only if pressure fluid is available in the suction chamber. It is thereby ensured that continued pressure fluid supply by the stepped piston will not occur when the suction chamber is pumped empty. Thus no vacuum may result in the return line and in the suction chamber so that air is not drawn into the braking pressure control circuit at the piston pump. It is a disadvantage in this known arrangement that the piston pump necessitates a comparatively complex structure.
It is known in high-pressure piston pumps for hydraulic systems as shown in DE-AS 23 36 118 and U.S. Pat. No. 2,845,029 to furnish the sealing means of the pump piston with an annular chamber from which a discharge channel leads either to the suction side of the pump or to the pressure fluid supply reservoir. This is meant to discharge leakage fluid and to prevent the ingress of air. These known high-pressure piston pumps are not suitable without modification for use on a braking pressure control device. When the discharge channel is led back to the reservoir, this necessitates further structure. When the discharge channel leads to the suction side, the ingress of air cannot be reliably avoided without taking special measures.
It is an object of the present invention to simplify the piston pump in a braking pressure control device of the type initially referred to and to ensure in doing so that the pressure in the return line does not fall below atmospheric pressure so that no air can enter through the piston seal of the piston pump.